1. Field of the Invention
This invention concerns a geothermal turbine which prevents aged deterioration and is capable of longer operating life, and a geothermal turbine having turbine components, such as blades, nozzle diaphragms and turbine rotors, are improved in terms of corrosion resistance or erosion resistance.
2. Description of the Related Art
Geothermal steam heated by underground magma is made available by drilling a well, and rotation power of a geothermal turbine is generated by directly introducing this steam as a working fluid into the turbine. Energy of the geothermal steam is changed into kinetic energy in a process of expanding the geothermal steam through turbine stage composed of nozzles and rotor blades, and this kinetic geothermal steam activates the rotor blades and thus generates power to rotate a turbine rotor shaft on which the rotor blades are mounted.
Since the geothermal steam is generated and heated by geothermal sources such as magma, the steam contains corrosive components such as hydrogen sulfide or sodium, scale components such as silicon dioxide or calcium, and solid particles such as sand, mud or ferrous oxide. Since the geothermal steam is fully or almost in a saturated state, the geothermal turbine is forced to operate under severe operating conditions due to high wetness of the steam inside the turbine. Moreover, the variety and consistency of the chemical composition of the geothermal steam, the size and the amount of solids carried into the geothermal turbine, and the steam condition of the geothermal steam, etc. differ by geothermal areas and wells in which the geothermal turbine is installed, and these factors even vary considerably over time even if the wells are same. These differences and aging make the design of the geothermal turbine more complicated.
Since the geothermal steam contains corrosive gas such as hydrogen sulfide, one concerns the factor shortening of the life of the geothermal turbine regarding the rotor blades, the nozzles and the rotor shaft, such as age deterioration and damage resulting from stress corrosion cracking or corrosion fatigue, in addition to corrosion of the materials. For example, generally, damage by the corrosion fatigue occurs as a result of three overlapping factors, namely, the environment, material and stress. It is difficult for the turbine components to avoid the overlapping of these three factors completely, because the rotor blades and the rotor shaft rapidly rotate, and consequently high stress occurs on the turbine components. Thus, subsequently, the material applicable to the geothermal turbine and the length used for geothermal turbine may be limited in a practical aspect, and the output of the geothermal turbine is restricted within a certain upper limit.
As a realistic measure of the geothermal turbine against corrosive environment peculiar to geothermal sources, suitable materials are selected in accordance with the results of evaluation examination of various candidate materials that are preliminarily held in an atmosphere of the geothermal steam, concurrently with setting the stresses on each part of the geothermal turbine at a level lower than that of an ordinary turbine for thermal power plant, etc. In other words, efforts to reduce the impact from the environment have been performed.
On the other hand, high wetness of the geothermal steam and solid particles carried over in the steam may give rise to the possibility of causing drain (liquid) erosion or particle erosion of the steam passage or sealing portion of each part of the turbine. The wet steam and solid particles, together with the corrosive gas contained in the geothermal steam, may cause synergy of erosion and corrosion and become a factor accelerating damage to the turbine. For this reason, as a measure for preventing the damage, a drain catcher is arranged at the exit of each turbine stage for discharging the water droplets and solid particles outside of the steam passage, and an erosion shielding is attached to the tip of the last stage blade.
In an actual geothermal turbine plant, in spite of such measures for preventing damage to the turbine against aging, severe degradation of each component in the geothermal turbine may nevertheless be observed. Thus, improvement of corrosion resistance and erosion resistance of each component of the geothermal turbine has been an important objective.
To convert thermal energy of geothermal steam into the rotational energy in the geothermal turbine with high efficiency, firstly, it is necessary to flow the working steam into the nozzles and the rotor blades of the geothermal turbine. Since the turbine rotor shaft rotates inside a stationary casing, a gap is required respectively between the rotating blade tips and the stationary nozzle diaphragm outer ring, and between the turbine rotor shaft and the nozzle diaphragm inner ring. It is important to minimize amount of leaked steam bypassing the steam passage through the gaps between stationary and rotating parts.
To minimize the leakage flow through the tips of the rotor blades, a conventional geothermal turbine is equipped with seal fins arranged on an inner periphery of an overhang of the nozzle diaphragm outer ring facing radially outside the shrouds of the rotor blades. This sealing equipment consists of multiple fins and forms a ring. The fins are extended radially inward and narrows the gap between the fins and the shrouds.
A steam sealing structure for preventing leakage of the steam on the rotor shaft is the same as mentioned above, that is, multiple fins arranged on an inner periphery of the nozzle diaphragm inner ring, which is a stationary part facing radially outside a radius of the turbine rotor shaft. These seal fins extend radially inward to narrow the gap between the nozzle diaphragm inner ring and the turbine rotor shaft. As for the sealing structure of the rotor shaft, in many cases, a high-and-low groove is arranged on the turbine rotor shaft to constitute so-called labyrinth seal configuration for additionally enhancing the effect of preventing steam leakage. This labyrinth-seal configuration is also applied to the seal structure of a gland packing portion in which the rotor shaft penetrates the casing.
In both cases mentioned above, it is indispensable for maintaining the efficiency of the geothermal turbine to maintain the steam seal effective without deterioration, wastage or dropout of these steam sealing structures in a geothermal environment.
Generally, for the geothermal turbine, an axial-flow turbine shown in FIG. 11 is adopted. That is, there are a plurality of stages composed of nozzles 1 and rotor blades 2. The rotating blades of the geothermal turbine are composed of a grouped blades structure, that is, a plurality of circumferentially-arranged blades 2a are connected by a shroud 3 for preventing vibration of the rotor blades 2 excited by high-speed steam discharged from the nozzles 1. These grouped blades effectively controls stress due to vibration generated by the steam rotating in the geothermal turbine within an acceptable level. These grouped blades are composed by fitting tenons 4 each arranged on, and coupled with, each of the rotor blades 2, respectively, into respective holes penetrating the shroud 3, for mortising the tenon 4 into the shroud 3, and thereby the plurality of blades 2a and the shroud 3 are connected together.
However, if a top of the tenon 4 of each blade tip 2a of the rotor blades 2 protrudes over the shroud 3, that is, protrudes raadially out of the shroud 3, it may be eroded and/or corroded by drain of the geothermal steam or the solid particles, and thus the life of the rotor blades 2 may become short. To prevent loss of the life of the rotor blade, as one example, a recessed tenon may be arranged so as not to protrude the tenon 4 over the upper surface of the shroud 3, and in addition, multiple seal fins 5b may be arranged on an overhang 5a of a nozzle diaphragm outer ring 5 opposed to the shroud 3, for forming a structure for preventing steam leakage at the blade tip. Thus, this structure for preventing steam leakage at the blade tip improves erosion and corrosion resistance of the tenon.
Moreover, for turbine stages operating in a severely corrosive environment such as in a geothermal turbine, fatigue strength of the rotating parts such as the blades and rotor materials is significantly decreased due to corrosion. The decrease of the fatigue strength of the blade materials, etc., directly affects the life of the blade relating vibration.
As mentioned above, the blade vibration stresses excited by steam forces are suppressed by the group structure. However, when the fatigue strength of the turbine materials significantly decreases under the corrosive environment, and thus making difficult to keep vibration stresses below the material fatigue limit, it is hard to avoid the risk of age damage coming from corrosion fatigue. To minimize this risk, the blade width may be preliminary arranged broader for increasing its rigidity. In this case, since the weight of the blade increases and thereby stresses of the blade fixation and rotor wheel increase, as a result, there occurs another aspect of risk such as stress corrosion cracking on the blade fixation, etc.
When a geothermal turbine is operated at low load, the last stage blade operates far from its aerodynamic design point with a substantially reduced output and pressure drop across the stage. This will cause nonsteadiness of the flow field with large back flow. The nonsteady turbulent flow around the last stage of the geothermal turbine acts on the blade as a strong exciting force, and thus the last stage blade needs a damping structure which is more effective than that of an ordinary turbine stage. For this reason, in many cases, in addition to the tenons 4 and the shroud 3 on the tip of the rotor blade 2, a coupling member called a lacing wire 6 is arranged in the middle of the blade as shown in FIG. 12. This lacing wire 6 is constituted by leading a wire through holes penetrating the blade and brazing them to each other, or by simply leading a wire through the holes as loose coupling. Generally, from the viewpoint of the damping effect of the blade, the loose coupling is superior to brazing.
In the thermal power turbines, a cover piece 7 may be arranged separately from the blades 2a, 2b at the blade tip as shown in FIG. 13, taking advantage of the excellent damping characteristics of the loose coupling. In this figure, the tenons 7a, 7b are protruded from opposite side faces of a rhombic cover piece 7, respectively, and one tenon 7a is inserted into a tenon hole bored at one blade tip for fixed joint, and the other tenon 7b is inserted into another tenon hole of the adjacent blade for loose coupling. Thus, this structure allows a small movement between the blades 2a, 2b and the cover piece 7. Sequential connection of adjacent blades 2a, 2b, . . . , through cover pieces loosely around the wheel constitutes continuous coupling of the blades 360 degrees and this continuous coupling provides excellent damping effect.
However, under a severe corrosive environment of the geothermal turbines, corrosive components deposited around the wire hole arranged or the tenon hole may easily become a trigger of stress corrosion cracking and corrosion and fatigue. Therefore, it is difficult to adopt this loose coupling connection structure to the geothermal turbine.
In the inlet stages of the geothermal turbines where the blade lengths are relatively short, natural frequencies of the grouped blades are chosen so as to avoid resonance with the nozzle passing frequency (NPF). NPF, which is the product of the number of nozzles and the rotational frequency of the shaft, is one of the excitation frequencies of the steam discharged from the nozzle. To avoid the resonance completely, this NPF is usually set above the lower modes of the natural frequencies of the grouped blades. The natural frequencies of such short blades are relatively high, and consequently, NPF must be set higher, resulted in a large number of nozzles.
Since the number of nozzles is inversely proportional to a size of the nozzles, the dimension size of the nozzle of these turbine stages becomes small. Arranging small nozzles for the turbine stages with short blade heights provides larger aspect ratio, that is the blade height divided by the blade width, and which might have better influence on stage efficiency. On the other hand, in this case, there is the disadvantage that resistance may be lowered against deterioration and damages inherent to the geothermal turbine, such as damages due to solid particle erosion, deterioration caused by corrosive components, and scale deposit.
The steam flow at the nozzle exit of geothermal turbines has a large circumferential velocity component. As the nature of geothermal turbines, when steam includes droplets (liquid components) and solid particles, a centrifugal force shifts them radially outward. As shown in FIG. 11, sealing fins 5b arranged at the overhang of the nozzle diaphragm outer ring may dam the particles centrifuged aside. However, because the strong circumferential velocity dominates in this portion, the droplets and the solid particles repeatedly circulate in a narrow pocket P surrounded by the outlet of the nozzle and the sealing fins 5b. Consequently, this pocket P may be greatly scooped out by erosion or the sealing fins 5b may drop out. The possibility of this risk becomes high when geothermal steam includes a lot of solid particles, and this is one of the main factors possibly adversely affecting the reliability of the geothermal turbine.
In order to avoid this disadvantage, some through holes 5c are arranged in a circumferential direction, connecting an exit of the stage with the pocket P surrounded by the nozzle exit and the sealing fins 5b. However, when the number of the holes 5b is small, the particles cannot be discharged completely, and conversely, when the number is large, an amount of an associated steam bypassing the rotor blade 2 increases, and thereby decreasing the efficiency of the turbine, which may cause a problem.
The problem of damage by the erosion or corrosion of a steam sealing portion of the geothermal turbine occurs not only at the blade tip, but also in a steam sealing portion between the turbine rotor shaft 8 and the nozzle diaphragm inner ring. Since the intensive rotational flow of the nozzle exit dominates also in this portion, the droplets and the solid particles may damage fins 9b arranged on a nozzle diaphragm side, and thereby the efficiency of the turbine often decreases.
Moreover, a labyrinth seal by arranging a high-and-low groove 8a on the rotor shaft 8, is adopted for raising sealing effectiveness, and that is the same steam sealing structure of gland packing portion in which the rotor shaft penetrates a turbine casing. However, the steam flows through this labyrinth portion with high velocity including droplets, solid particles and corrosive components as mentioned above, and thus the high-and-low groove 8a will be shaved off over time.
In any event, maintaining long-term reliability or extension of a life span of the steam sealing performance between a rotating portion and a stationary portion is one of the issues which should be solved for a geothermal turbine.
On the other hand, for the turbine inlet stages, relatively small nozzles, having a throat width at the nozzle exit of 5 to 8 millimeters, are adopted. Usually, a strainer is installed in the inlet portion of the geothermal turbine, so as not to entrain large solid particles into the turbine. On the other hand, depending on conditions of a well of the geothermal site, such a strainer frequently becomes plugged up, and cleaning of the strainer is often required. Thus, unavoidably, course or large meshes, for example, two meshes per inch, may be used. These rough meshes may cause a partial blockage of the nozzle throat due to solid particles passed through the meshes, and thereby the output of the turbine may decreases significantly. And even if the nozzle throat is not blocked, the throat becomes narrow due to scale deposits on the nozzle surfaces, and thus the output may decrease.
Furthermore, when corrosion or pitting corrosion occurs on a surface of the nozzle, the efficiency may be extensively affected. Especially, corrosive elements are active at steam conditions of inlet stages of the geothermal turbine, and thus this portion is easily affected by corrosion and/or pitting corrosion (erosion), whereby the surface roughness of both nozzle and blade may become severely deteriorated. Since the efficiency drop due to deteriorated surface roughness of nozzle and blade is related to a relative value of the roughness against blade size, the smaller the nozzle or blade is, the more extensive the influence becomes.
In addition, for a small nozzle, there is also a problem of damage on the surface of the nozzle due to solid particles. Since the rate of curvature becomes large as for a smaller nozzle, the solid particles cannot follow the rapid turning of the stream because of inertia and collide with nozzle surfaces. Consequently, a thin outlet portion of the nozzle profile may be damaged and the nozzle profile may be extensively deformed, whereby the efficiency may decline. When the damage is large, an accompanying exciting force against the rotor blades becomes excessive, and the reliability of the rotor blades itself is also affected.
Moreover, since geothermal resources are limited, partial load operation is occasionally carried out during nighttime hours, when electric power demand is lower. Such a style of operation is required especially when the underground resource of the steam tends to be exhausted due to long-running service. At low load, the last stage blade operates far from its aerodynamic design point with a substantially reduced output and pressure drop across the stage. This will cause the flow field to be unsteady with large back flow. This unsteady reversed flow spreads widely as the load decreases, and the exciting forces on the blade become more intensive, resulting in large intensive vibrating stresses in the blade.
If the vibration stresses on a stress concentrated portion, such as blade connection, corrosion pits or erosion pits, exceed the fatigue strength of materials which has been decreased under the corrosive environment, there arises the possibility of cracking as the worst case. Thus, the reliability of geothermal turbines used for cycling load operation depends on how the vibration stresses of the last stage blade could be suppressed applying effective damping structure.